Figure 4. Plots located on upper- (top) and lower-slope (bottom) positions in the same grower-owned plantation.
In soil/site studies with forest stands in Ohio, the author has found that indirect effects of topographic factors on growth of trees can be very substantial, particularly on the upper portions of steeper areas having southwest-facing slopes (Brown and Stires, 1984; Brown and Marquard, 1988; Brown and Duncan, 1990). As indicated in Table 1, the average percent distance to ridge (slope position) for plots was at approximately mid-slope with a distribution of plots from ridge (0 percent) to bottomland (100 percent positions). Although there were some plots on steeper areas (maximum slope of 26 percent), surface topography of study plots can be characterized generally as gently sloping to flat, averaging 7.7 percent. Under those conditions, effects of azimuth/aspect (direction an area faces) is usually minimal, and this is reflected in the fact that there were no significant correlations between azimuth and any of the individual tree characteristics evaluated (Table 2). However, within individual plantings there were some graphic illustrations of the effects of azimuth (Figure 3) and slope position (Figure 4).
Figure 4. Plots located on upper- (top) and lower-slope (bottom) positions in the same grower-owned plantation.
For slope percent, there were relatively small but statistically significant correlations with five of the seven tree characteristics; however, all of those relationships are positive, indicating that tree growth and foliage characteristics improve as slope percent increases (Table 2). This is opposite of what has been commonly found in other soil/site studies in Ohio and is probably reflective of the generally low slope percents on plots where increased surface and subsurface runoff of precipitation could improve soil aeration for sites having the relatively fine textured soils which were characteristic on many of the plots sampled (Tables 1 and 2). However, there were noticeable effects on growth and other tree characteristics in plantings where there were was a wide range in slope percents (Figure 5).
Slope shape - whether it is concave, even/flat, or convex shaped - can have a major indirect topographic influence on survival and tree performance, and effects may be different depending on other topographic/soil factor combinations. On coarser-textured soils and/or drier upland positions, concave-shaped surfaces can have beneficial effects by helping collect or slow down surface runoff of precipitation; however, in bottomland areas or with finer textured soils, collection of water in concave-shaped areas can keep soils saturated to the point were survival and growth can be seriously affected. For the 80 plots in this study, the average slope shape, 1.8, was near the flat/even rating, with approximately equal numbers of plots having concave (1) and convex (3) configurations (Table 1); however, the concave-shaped plots often occurred in bottomland positions and often with finer-textured soils.
As a result, there were relatively small but statistically significant correlations which indicated that survival, height growth, and foliage color were better on plots having flat/even to convex-shaped surfaces where surface runoff would be greater (Table 2, Figure 6 ).
